[0001] The present invention relates to an FM-CW radar apparatus that uses a frequency-modulated
(FM) continuous wave (CW) as a signal for transmission and, more particularly, to
a DBF radar apparatus that performs digital beam forming.
[0002] A conventional DBF radar apparatus comprises: a transmitter section for radiating
an electromagnetic wave as a transmit signal; an array antenna comprising a plurality
of antenna elements for receiving, as a received signal, the electromagnetic wave
reflected by an object; a plurality of input terminals connected to the respective
antenna elements; an output terminal which is selectively connected by a switching
means to one of the plurality of input terminals; and a receiver section in which
the received signal obtained from the output terminal is downconverted by using a
portion of the transmitted signal and the resulting difference signal representing
the difference between the transmitted signal and the received signal is converted
into a digital signal, wherein the distance to or the relative velocity of the object
is detected by applying prescribed processing to the digital signal obtained from
the receiver section (refer, for example, to Publication of Japanese Unexamined Patent
Application No. 11-160423).
[0003] It is an object of the present invention to provide a DBF radar apparatus that can
accomplish highly accurate measurements.
[0004] According to the radar apparatus of the present invention, a plurality of antennas
are divided into a plurality of groups, while setting at least one of the plurality
of antennas as a common antenna, and the apparatus includes a plurality of selector
switches each for sequentially and selectively switching output terminals of the plurality
of antennas and the common antenna contained in a corresponding one of the groups
for connection to an input terminal. The apparatus further includes a plurality of
channels each having an A/D converter and a downconverting section for downconverting
a received signal by using a portion of a transmitted signal, that is, the received
signal that has been input from each of the antennas to the input terminal is downconverted
and A/D converted in a corresponding one of the plurality of channels, and prescribed
processing is applied in a digital signal processing section to detect the distance
to or the relative velocity of the object.
[0005] According to the present invention, as at least one of the plurality of antennas
is shared between the plurality of channels, highly accurate measurements can be accomplished
by applying corrections.
[0006] The above object and features of the present invention will be more apparent from
the following description of the preferred embodiments with reference to the accompanying
drawings, wherein:
Figs. 1A, 1B, and 1C are diagrams for explaining the principle of FM-CW radar when
the relative velocity with respect to target is 0;
Figs. 2A, 2B, and 2C are diagrams for explaining the principle of FM-CW radar when
the relative velocity with respect to target is v;
Fig. 3 is a diagram showing one configuration example of the FM-CW radar;
Fig. 4 is a diagram showing the basic configuration of a digital beam forming (DBF)
radar;
Fig. 5 is a diagram showing a modified example of the digital beam forming (DBF) radar;
Fig. 6 is a diagram showing a signal output from a switching signal generator and
a waveform for explaining antenna switching operation;
Fig. 7 is a diagram showing the waveforms of downconverted signals;
Fig. 8 is a diagram showing an embodiment of a radar apparatus according to the present
invention;
Fig. 9 is a diagram showing the configuration of a branching means;
Fig. 10 is a diagram showing a radar apparatus according to the present invention
for the case where the number of channels is 3; and
Figs. 11A and 11B are diagrams for explaining how corrections are applied.
[0007] Before describing the radar apparatus of the present invention, the principle of
FM-CW radar and the principle of DBF radar will be described.
[0008] An FM-CW radar measures the distance to a target object, such as a vehicle traveling
ahead, by transmitting a continuous wave frequency-modulated, for example, in a triangular
pattern. More specifically, the transmitted wave from the radar is reflected by the
vehicle ahead, and the reflected signal is received and mixed with the transmitted
signal to produce a beat signal (radar signal). This beat signal is fast Fourier transformed
to analyze the frequency. The frequency-analyzed beat signal exhibits a peak at which
power becomes large in correspondence with the target. The frequency corresponding
to this peak is called the peak frequency. The peak frequency carries information
about distance, and the peak frequency differs between the rising portion and falling
portion of the triangular FM-CW wave due to the Doppler effect associated with the
relative velocity with respect to the vehicle ahead. The distance and relative velocity
with respect to the vehicle ahead are obtained from the peak frequencies in the rising
and falling portions. If there are more than one vehicle traveling ahead, a pair of
peak frequencies in the rising and falling portions is generated for each vehicle.
Forming pairs of peak frequencies in the rising and falling portions is called pairing.
[0009] Figs. 1A, 1B, and 1C are diagrams for explaining the principle of the FM-CW radar
when the relative velocity with respect to the target is 0. The transmitted wave is
a triangular wave whose frequency changes as shown by a solid line in Fig. 1A. In
the figure, f0 is the center frequency of the transmitted wave, Δf is the FM modulation
width, and Tm is the repetition period. The transmitted wave is reflected from the
target and received by an antenna; the received wave is shown by a dashed line in
Fig. 1A. The round trip time T to and from the target is given by T = 2r/C, where
r is the distance (range) to the target and C is the velocity of radio wave propagation.
[0010] Here, the received wave is shifted in frequency from the transmitted signal (i.e.,
produces a beat) according to the distance between the radar and the target.
[0011] The frequency component fb of the beat signal can be expressed by the following equation.
where fr is the frequency due to the range (distance).
[0012] Figs. 2A, 2B, and 2C, on the other hand, are diagrams for explaining the principle
of the FM-CW radar when the relative velocity with respect to the target is v. The
frequency of the transmitted wave changes as shown by a solid line in Fig. 2A. The
transmitted wave is reflected from the target and received by the antenna; the received
wave is shown by a dashed line in Fig. 2A. Here, the received wave is shifted in frequency
from the transmitted signal (i.e., produces a beat) according to the distance between
the radar and the target. In this case, as the relative velocity with respect to the
target is v, a Doppler shift occurs, and the beat frequency component fb can be expressed
by the following equation.
where fr is the frequency due to the distance, and fd
the frequency due to the velocity.
[0013] The symbols in the above equation have the following meanings.
fb: Transmit beat frequency
fr: Range (distance) frequency
fd: Velocity frequency
f0: Center frequency of transmitted wave
Δf: Frequency modulation width
Tm: Period of modulation wave
C: Velocity of light (velocity of radio wave)
T: Round trip time of radio wave to and from target object
r: Range (distance) to target object
v: Relative velocity with respect to target object
[0014] Fig. 3 is a diagram showing one configuration example of the FM-CW radar. As shown,
a modulating signal generator MOD applies a modulating signal to a voltage-controlled
oscillator VCO for frequency modulation, and the frequency-modulated wave is transmitted
out from a transmitting antenna AT, while a portion of the transmitted signal is separated
and fed into a mixer MIX. The signal reflected from the target is received by a receiving
antenna AR, and the received signal is mixed in the mixer MIX with the output signal
of the voltage-controlled oscillator VCO to produce a beat signal. The beat signal
is passed through a filter F, and is converted by an A/D converter into a digital
signal; the digital signal is then supplied to a digital signal processor DSP where
signal processing, such as a fast Fourier transform, is applied to the digital signal
to obtain the distance and the relative velocity.
[0015] Digital beam forming is a technology in which signals received by an array antenna
with a plurality of receiving antennas are each A/D converted and fed to a digital
signal processing section and adjustments of beam scanning and sidelobe characteristics,
etc. are made in the digital signal processing section.
[0016] A DBF radar accomplishes the function of phase shifters in a phased-array antenna
radar by digital signal processing; the basic configuration of the DBF radar is shown
in Fig. 4.
[0017] As shown, a modulating signal generator MOD applies a modulating signal to a voltage-controlled
oscillator VCO for frequency modulation, and the frequency-modulated wave is transmitted
out from a transmitting antenna AT, while a portion of the transmitted signal is separated
and fed into a plurality of mixers MIX1 to MIXn. Signals reflected from objects are
received by a plurality of receiving antennas AR1 to ARn, and the received signals
from the respective receiving antennas are fed via respective amplifiers AMP1 to AMPn
into the respective mixers MIX1 to MIXn where the received signals are each mixed
with the output signal of the voltage-controlled oscillator VCO to produce beat signals.
[0018] The beat signals thus produced are passed through respective filters F1 to Fn and
are converted by respective A/D converters A/D1 to A/Dn into digital signals which
are supplied to a digital signal processor DSP. In the DSP, phase shifting (PH-SFT)
is applied to the digital signal from each channel, and all the channels are combined.
[0019] The feature of DBF is that a plurality of beams can be created in a single acquisition
process because, once the signals from all the receiving antennas are taken in as
digital signals, the beam can be synthesized in any desired direction based on the
digital signals.
[0020] On the other hand, Publication of Japanese Unexamined Patent Application No. 11-160423
discloses a DBF radar apparatus in which one amplifier, mixer, filter, etc., provided
for each antenna in the DBF radar apparatus shown in Fig. 4, are consolidated into
one. To make understanding of the present invention easy, such a radar apparatus will
be explained with reference to Fig. 5.
[0021] In Fig. 5, reference numeral 1 is a transmitter section, 2 is an antenna section,
3 is a selector switch section, 4 is a receiver section, and 5 is a digital signal
processing section. In the transmitter section 1, the modulating signal generator
MOD applies a modulating signal to the voltage-controlled oscillator VCO for frequency
modulation, and the frequency-modulated wave of f0±Δf/2 is transmitted out from the
transmitting antenna AT, while a portion of the transmitted signal is separated and
fed into a mixer 42 in the receiver section 4. Signals reflected from objects are
received by the plurality of receiving antennas AR1 to ARn in the antenna section
2, and the received signals from the respective receiving antennas are fed via the
selector switch section 3 into the receiver section 4.
[0022] The receiving antennas comprise n receiving antennas AR1 to ARn corresponding to
the first channel ch1 to the nth channel chn. The selector switch section 3 comprises
n terminal outputs t1 to tn corresponding to the n antennas and one input terminal
T which is connected to one of the terminals t1 to tn, the connection being switched
cyclically by a switching signal supplied from a switching signal generator 46. The
signals received by the receiving antennas are each amplified by an amplifier 41 and
mixed in the mixer 42 with a portion of the transmitted signal. With this mixing,
the received signal is downconverted to produce a beat signal which is the difference
between the transmitted signal and the received signal. The beat signal is then passed
through an amplifier 43 and a filter 44 and input to an A/D converter 45 where it
is converted into a digital signal in synchronism with the timing of the switching
signal output from the switching signal generator 46. The digital signal processing
section (DSP) 5 performs digital beam forming by taking the digital beat signal from
the A/D converter 45 as an input.
[0023] Connection switching is performed using the signal output from the switching signal
generator 46. As shown in Fig. 6, this switching signal is a clock signal of frequency
fsw, and channel switching occurs at each of the rising and falling edges of the switching
signal with the frequency of fsw, as shown by p1 to pn on ch1 to chn. With this switching,
ch1 is selected during p1 (time t1 to t2), ch2 is selected during p2 (time t2 to t3),
and in like manner chn is selected during pn (time tn to tn+1) in sequence for connection
to the amplifier 41. The time interval of each of the periods p1 to pn is the same,
i.e., T1, and the channels are switched one after another with the period T1, as shown
in Fig. 6.
[0024] The waveforms of the signals downconverted by the mixer 42 are shown in Fig. 7. In
Fig. 7, the time is plotted along the abscissa and the voltage along the ordinate.
Vertical lines are drawn with a spacing equal to the channel switching period T1 with
which the selector switch performs switching. The figure shows the case where the
number of receive channels n = 8; as shown, the phases of the beat signals received
from the respective channels ch1 to ch8 are slightly shifted from each other. The
eight curves show the beat signals produced by downconverting the signals received
on the respective channels. In this embodiment, as the channel switching is performed
with the period T1 based on the signal output from the switching signal generator
46, the beat signals output from the mixer 42 have the waveforms shown by thick lines
in Fig. 7. Each beat signal is passed through the amplifier 43 and the filter 44 and
input to the A/D converter 45. In the A/D converter 45, the beat signal is converted
into a digital signal which is supplied to the digital signal processor (DSP) 5. In
the DSP, phase shifting (PH-SFT) is applied to the digital signal from each channel,
and all the channels are combined.
[Embodiment 1]
[0025] Fig. 8 shows an embodiment of the radar apparatus of the present invention. This
radar apparatus differs from the radar apparatus of Fig. 5 in that a plurality of
channels each having a downconverting section are provided in the receiver section
4, with provisions made to share at least one of the plurality of antennas between
the plurality of channels.
[0026] In Fig. 8, two downconverting sections are provided, and reference numeral 1 indicates
the transmitter section, 2 the antenna section, 3 the selector switch section, 4 the
receiver section, and 5 the digital signal processing section (DSP). In the transmitter
section 1, the modulating signal generator MOD applies a modulating signal to the
voltage-controlled oscillator VCO for frequency modulation, and the frequency-modulated
wave of f0±Δf/2 is transmitted out from the transmitting antenna AT, while a portion
of the transmitted signal is separated and fed into mixers 42A and 42B in the receiver
section 4. Signals reflected from objects are received by the plurality of receiving
antennas in the antenna section 2, and the received signals from the respective receiving
antennas are fed via the selector switch section 3 into the receiver section 4.
[0027] The receiving antennas are divided into two groups, group A consisting of n receiving
antennas AR11 to AR1n and group B consisting of n receiving antennas AR21 to AR2n.
The antenna section 2 further includes a common antenna AR0 connected to a branching
means hereinafter described.
[0028] The selector switch section 3 includes output terminals t11 to t1n of the antennas
in the group A, output terminals t21 to t2n of the antennas in the group B, and terminals
t01 and t02 of the branching means 31. SW1 is a selector switch for the group A, by
which the connection to the terminals t11 to t1n and t01 is sequentially switched
with a prescribed period to sequentially input the received signals from the respective
antennas AR11 to AR1n and the common antenna AR0 into the receiver section. Likewise,
SW2 is a selector switch for group B, by which the connection to the terminals t21
to t2n and t02 is sequentially switched with a prescribed period to sequentially input
the received signals from the respective antennas AR21 to AR2n and the common antenna
AR0 into the receiver section.
[0029] The branching means 31, as shown in Fig. 9, includes a selector switch SW0 by which
the output of the common antenna AR0 is switched between the terminal t01 and the
terminal t02. Alternatively, a hybrid circuit may be used as the branching means.
[0030] On the other hand, the receiver section 4 comprises two channels: channel A includes
an amplifier 41A, a mixer 42A as a downconverter, an amplifier 43A, a filter 44A,
and an A/D converter 45A, and channel B includes an amplifier 41B, a mixer 42B as
a downconverter, an amplifier 43B, a filter 44B, and an A/D converter 45B.
[0031] In Fig. 8, the number of channels is 2, but three or more channels may be provided.
In that case, the receiving antennas are divided into the same number of groups as
there are channels, and the number of common antennas and branching means is increased
as the number of channels increases.
[0032] The received signals input from the antennas AR11 to AR1n and the common antenna
AR0 in the antenna section 2 are sequentially and selectively switched by SW1 for
input to the channel A in the receiver section 4, and likewise, the received signals
input from the antennas AR21 to AR2n and the common antenna AR0 are sequentially and
selectively switched by SW2 for input to the channel B in the receiver section 4.
[0033] The switching operations of SW1 and SW2 are controlled by the clock signal of frequency
fsw shown, for example, in Fig. 6, that is output from the switching signal generator
46. Switching from one antenna to the next occurs at each of the rising and falling
edges of the clock signal, that is, the channel switching is performed with the period
T1.
[0034] The signal output from the amplifier 41A is fed into the mixer 42A acting as the
downconverter, where the signal is downconverted by being mixed with the output signal
of the voltage-controlled oscillator VCO, and a beat signal is thus produced. The
waveform of the downconverted signal is the same as that shown in Fig. 7. The beat
signal whose waveform is shown by a thick line in Fig. 7 is then passed through the'
amplifier 43A and the filter 44A and input to the A/D converter 45A. In the A/D converter
45A, the beat signal is converted into a digital signal which is supplied to the digital
signal processor (DPS) 5.
[0035] The received signals input from the antennas AR21 to AR2n and the common antenna
AR0 in the antenna section 2 are sequentially and selectively switched by SW2 for
input to the channel B in the receiver section 4. The remainder of the operation is
the same as that described for the channel A.
[0036] As shown in Fig. 8, of the plurality of antennas, the common antenna AR0 is shared
between the two channels A and B each having a downconverter. In the embodiment of
Fig. 8, one antenna is shared, but two or more antennas may be shared.
[0037] Next, the operation of the embodiment of the present invention shown in Fig. 8 will
be described. The received signal obtained via the switch SW1 is downconverted and
A/D converted on the channel A and input to the DSP 5 for processing. Likewise, the
received signal fed via the switch SW2 is downconverted and A/D converted on the channel
B and input to the DSP 5 for processing.
[0038] In a radar apparatus, the characteristics of the mixers, etc. vary depending on the
surrounding environment such as temperature, and an accurate measurement may not be
accomplished. According to the present invention, since corrections are applied in
such cases, the accuracy of measurement can be enhanced.
[0039] Referring to Fig. 10 and Figs. 11A and 11B, a description will be given, below, of
how corrections are applied in the radar apparatus of the present invention. Fig.
10 shows the case where the number of channels is 3 in the radar apparatus of the
present invention. Antennas (1) to (9) are divided into three groups (A), (B), and
(C), and the receiver section comprises three channels (A), (B), and (C). In the figure,
the number of antennas in each group is three or four, but the invention is not limited
to this particular number.
[0040] Figs. 11A and 11B show how corrections are applied. Fig. 11A shows the received signals
fed via the switches SW1 to SW3 to the respective channels (A), (B), and (C) for the
case where corrections are applied. As shown in Fig. 11A, the received signals from
the antennas (1) to (3) are input to the channel (A), the received signals from the
antennas (3) to (6) are input to the channel (B), and the received signals from the
antennas (6) to (9) are input to the channel (C).
[0041] Here, the received signal from the antenna (3) is input to both the channels (A)
and (B), and the received signal from the antenna (6) is input to both the channels
(B) and (C).
[0042] When a designated one of the plurality of antennas is shared between each of the
plurality of channels as shown above, correlations of the phase and amplitude characteristics
between the respective channels can be obtained by producing beat signals on different
channels for the same received signal, and corrections can be applied based on the
correlations. As a result, the accuracy of measurement does not degrade even when
the surrounding environment changes.
[0043] On the other hand, Fig. 11B shows the received signals fed via the switches SW1 to
SW3 to the respective channels (A), (B), and (C) for the case where corrections are
not applied. As shown in Fig. 11B, no antennas are shared between the respective channels.
[0044] Since, usually, the surrounding environment changes sufficiently slowly compared
with the observation time, data collection for inter-channel corrections need not
be performed for each observation. Accordingly, corrections can be applied when a
temperature change is detected, or at predetermined intervals of time.
[0045] When only one channel comprising the downconverter mixer, etc. is provided for the
plurality of antenna arrays as in the configuration of Fig. 5, switching must be performed
at high speed within the measuring sector so as not to disrupt the correlation between
the respective antenna arrays, and the resulting signals are as shown in Fig. 7. If
each signal waveform resulting from such high-speed switching is to be faithfully
input to the A/D converter, the bandwidth becomes much wider than the actual beat
signal bandwidth, and this can result in a degradation of the S/N ratio.
[0046] A method for reducing the number of channels to be switched within one data acquisition
sector (measuring sector) for the same number of antenna arrays is disclosed, for
example, in Publication of Japanese Unexamined Patent Application No. 11-31168. In
this case, after first setting the reference channel, combinations must be considered,
and processing for synthesis must be performed using data from a plurality of sectors
(a plurality of FMs).
[0047] When applications as automotive radars are considered, there are many target objects
having large level variations and large relative velocity, and it is difficult to
ensure simultaneity; besides, problems associated with measurement errors or sensitivity
are highly likely to occur. Even in applications where such problems do not occur,
if the number of observation sectors can be reduced, the number of detection results
(detected information) that can be obtained within the total observation time increases;
as a result, it becomes possible to apply processing such as averaging, and this serves
to enhance the detection accuracy and sensitivity.
[0048] However, if the number of channels is simply increased (for example, the configuration
of Fig. 4), the system becomes more susceptible to changes in the surrounding environment
because of the temperature and frequency characteristics of the mixer, etc. provided
in each channel, and the accuracy degrades as the phase and amplitude relations between
the respective channels change. To solve this problem, in the present invention, one
antenna array is shared between a plurality of channels, and beat signals are produced
on different channels for the same received signal, based on the analysis results
of which the correlations of the phase and amplitude characteristics between the respective
receive channels are obtained; this configuration serves to prevent accuracy degradation
even when the surrounding environment changes.